Hibernate Converter Causes SQLException

Question

In my Hibernate entity class I have a Boolean that I want to be true for certain characters in a column of VARCHAR2(1). But my code causes a SQLException: Fail to convert to internal representation. Can someone see what I'm doing wrong? My code is as follows:

In my entity I have:

...

    private Boolean flag;

    @Column(length = 1, name = "FLAG")
    @Convert(converter = FlagToBooleanConverter.class)
    public Boolean getFlag()
    {
        return flag;
    }

    void setFlag(Boolean flag)
    {
        this.flag = flag;
    }

...

@Converter
class FlagToBooleanConverter implements AttributeConverter<Boolean, String>
{
    @Override
    public Boolean convertToEntityAttribute(String value)
    {
        return (value.equals("X") || value.equals("Y"));
    }

    @Override
    public Boolean convertToDatabaseColumn(String value)
    {
        // My class should only read from the DB so I don't need this but I have to override it
        return null;
    }
}

UPDATE: Feels kind of hacky, but at the moment, to get the job down I've been using:

    private String flag;

    @Column(length = 1, name = "FLAG")
    String getFlag()
    {
        return flag;
    }

    void setFlag(String flag)
    {
        this.flag = flag;
    }

    @Transient
    public Boolean isFlag()
    {
        return (value.equals("X") || value.equals("Y"));
    }